The external quantum efficiency of radiation-emitting organic electronic devices is defined by the ratio of photons emitted into the environment to the electron-hole pairs (excitons) injected. The external quantum efficiency can be described by the following formula:ηext=γ×ηS/T×qeef×θout.
In this formula, γ is the charge carrier equilibrium factor, which indicates the ratio between electrons and holes injected, which together form excitons. The factor ηS/T indicates the proportion of excitons which can break down radiatively and is referred to as exciton formation efficiency or as singlet/triplet ratio. In fluorescent compounds, this value is limited to a maximum of 25% because of the spin selection rule. Through the use of phosphorescent compounds, it is theoretically possible to quadruple this value. qeff denotes the effective internal quantum efficiency, which describes the proportion of excitons which combine radiatively. The factor ηout indicates what proportion of the photons generated can leave the radiation-emitting organic electronic device and is referred to as emission efficiency. The external quantum efficiency ηext of radiation-emitting organic electronic devices, especially of organic light-emitting diodes (OLEDs) is thus determined both by the nature of the emitter used and to a large degree by optical parameters.
The optical effects which reduce the efficiency of light emission from the device include coupling losses into the substrate, excitation of waveguide modes in various layers of a radiation-emitting organic electronic device, absorption losses and losses through excitation of plasmons (electron density vibrations) at metallic electrodes. The consequence of this is that only about 20% of the photons generated are emitted to the outside, meaning that the external quantum efficiency, assuming an effective internal quantum efficiency of 100%, i.e., even when phosphorescent triplet emitters are used, is limited to about 20%. If fluorescent singlet emitters are used, the theoretically achievable external quantum efficiency also decreases together with the effective internal quantum efficiency to about 5%. About 30% of the photons generated in a light-emitting layer of a radiation-emitting organic electronic device become unavailable for emission into the environment as a result of the generation of surface plasmons at a metallic electrode.
There are known measures for increasing the external quantum efficiency, for example, by more efficiently emitting the light conducted through a substrate. For this purpose, for example, films having scattered particles or films having surface structures, for instance micro-lenses or prisms, are used on the outside of the substrate. It is also known that direct structuring of the outside of the substrate can be provided, or scattered particles can be introduced into the substrate. Some of these approaches, for example, the use of scattering films, are already being used commercially and can be scaled up in terms of the emission area especially in the case of the OLEDs executed as lighting modules. However, these approaches for light emission have the significant disadvantages that the emission efficiency is limited to about 60%-70% of the light conducted within the substrate, and that the appearance of the OLED is significantly affected since the layers or films applied result in a milky, diffusely reflective surface. Disadvantages of these processes are also that costs are sometimes high and production processes complicated. There are known measures for increasing internal light emission, for example, by disposing scattering layers between the substrate and an ITO anode, or roughening the substrate surface above one ITO anode.